Electrical circuits on microchips are becoming more complex and are used in many more and more different types of circuits. For example, a circuit which is principally a digital circuit may have an analog component(s) or subcircuit(s) which comprises a part of the circuit. In such case analog signals must be converted to digital signals which means that it must contain an analog to digital (A to D) converter.
Of course, circuits that are formed on chips must be tested. In the past a conventional form of testing has been to apply signals from an external source. This technique however, has several drawbacks. First, and very significantly many of the analog subcircuits do not have external entry points for signals to be applied, and thus testing from external sources becomes difficult when it is desired to test the individual subcircuits as well as the entire circuitry on the chip. Also, when a circuit contains analog circuits and an A to D converter, conventional prior art practice has been to apply analog signals to the analog circuit or to the A to D converter and use the analog signals in a predetermined fashion to test the converter. This presents problems since precisely picking up test points on an analog signal can become difficult and may lead to errors. Thus it is desired to provide an improved test method and circuit or testing A to D converters especially where the A to D converters are contained as subcircuits in larger circuitry, and more particularly when the larger circuitry includes significant amounts of digital circuitry.